Abstract
Abstract. Nitrous oxide (N2O) fluxes measured using the eddy-covariance method capture the spatial and temporal heterogeneity of N2O emissions. Most closed-path trace-gas analyzers for eddy-covariance measurements have large-volume, multi-pass absorption cells that necessitate high flow rates for ample frequency response, thus requiring high-power sample pumps. Other sampling system components, including rain caps, filters, dryers, and tubing, can also degrade system frequency response. This field trial tested the performance of a closed-path eddy-covariance system for N2O flux measurements with improvements to use less power while maintaining the frequency response. The new system consists of a thermoelectrically cooled tunable diode laser absorption spectrometer configured to measure both N2O and carbon dioxide (CO2). The system features a relatively small, single-pass sample cell (200 mL) that provides good frequency response with a lower-powered pump ( ∼ 250 W). A new filterless intake removes particulates from the sample air stream with no additional mixing volume that could degrade frequency response. A single-tube dryer removes water vapour from the sample to avoid the need for density or spectroscopic corrections, while maintaining frequency response. This eddy-covariance system was collocated with a previous tunable diode laser absorption spectrometer model to compare N2O and CO2 flux measurements for two full growing seasons (May 2015 to October 2016) in a fertilized cornfield in Southern Ontario, Canada. Both spectrometers were placed outdoors at the base of the sampling tower, demonstrating ruggedness for a range of environmental conditions (minimum to maximum daily temperature range: −26.1 to 31.6 °C). The new system rarely required maintenance. An in situ frequency-response test demonstrated that the cutoff frequency of the new system was better than the old system (3.5 Hz compared to 2.30 Hz) and similar to that of a closed-path CO2 eddy-covariance system (4.05 Hz), using shorter tubing and no dryer, that was also collocated at the site. Values of the N2O fluxes were similar between the two spectrometer systems (slope = 1.01, r2 = 0.96); CO2 fluxes as measured by the short-tubed eddy-covariance system and the two spectrometer systems correlated well (slope = 1.03, r2 = 0.998). The new lower-powered tunable diode laser absorption spectrometer configuration with the filterless intake and single-tube dryer showed promise for deployment in remote areas.
Highlights
The concentration of N2O in the atmosphere is rising and is of concern as N2O has 298 times the global warming potential of CO2 (IPCC, 2013)
The main drivers of microbial N2O emissions are soil conditions, but distal drivers such as nitrogen fertilization, dry–wet, and freeze–thaw cycles exert control and result in N2O fluxes (FN2O) that are highly variable in time (“hot moments”) and in space (“hot spots”; Groffman et al, 2009; Molodovskaya et al, 2012)
The field trial of the new tunable diode laser absorption spectrometer (TDLAS)-TE analyzer with the optimized sampling system consisting of a vortex intake, singletube drier, and lower-power pump demonstrated the ability of this analyzer to operate continuously through several field seasons with minimal maintenance with the frequency response needed for EC measurements
Summary
The concentration of N2O in the atmosphere is rising and is of concern as N2O has 298 times the global warming potential of CO2 (IPCC, 2013). The main drivers of microbial N2O emissions are soil conditions (e.g., soil oxygen, carbon, and nitrogen levels), but distal drivers such as nitrogen fertilization, dry–wet, and freeze–thaw cycles exert control and result in N2O fluxes (FN2O) that are highly variable in time (“hot moments”) and in space (“hot spots”; Groffman et al, 2009; Molodovskaya et al, 2012) This sporadic nature of FN2O necessitates continuous measurements covering areas large enough to capture the spatial heterogeneity of the fluxes to sufficiently quantify total emissions from agricultural systems (Flechard et al, 2007; Jones et al, 2011; Shurpali et al, 2016). Locations with agricultural N2O emissions are often rural areas where it may be impractical and expensive to install permanent power infrastructure for long-term FN2O EC flux measurements
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